It is known that some properties of many kinds of polymers (e.g., various rubbers, polyesters, polystyrenes, polyepoxides, etc.) can be enhanced, and their costs reduced, by the addition of various inexpensive layered silicates or clays, such as montmorillonite (MMT). However, there are issues of compatibility between the use of such polymers and these polar inorganic solids. Thus, it is known in the art to modify these polar inorganic solids with organic chemicals to enhance their compatibility with organic polymers, and hence, to obtain composites with improved properties.
Among the advances in organically-modifying these inorganic solids are the use of quaternary ammonium cations having a relatively long hydrocarbon substituent (e.g., +NR3—C16). These cations are particularly well suited for use as swelling agents and modifiers of MMT. The —NR3+ function of the modifier cation becomes ionically bound to the clay, disrupts its layered structure, and thus enables the entry (intercalation) of polymers in-between the clay's layers (galleries). The relatively long hydrocarbon substituent of the modifier renders the clay essentially organophilic and enhances the compatibility of the clay with synthetic polymers such as those above. Exfoliation occurs when the layers of the clay are essentially completely disrupted and the individual layers separate.
Blends of layered clays with polymers are often termed “nanocomposites” because at least one of the dimensions of the clay's layers is in the nanometer dimension. The properties of such nanocomposites are often superior to the virgin polymer. Considerable research and development is being carried out toward the preparation, characterization and testing of such nanocomposites.
Shuo et al., J. Appl. Polym. Sci., 94, 534, (2004) alleges the preparation of polyurethane nanocomposites by the use of an organically-modified montmorillonite (OmMMT). The MMT is modified by 1,6-hexamethylene diamine (NH2—(CH2)6—NH2). The reference discloses that this OmMMT is used as a chain extender to replace part of the conventional 1,2-propane diamine (NH2—CH2CH(NH2)CH3) chain extender for the preparation of polyurethanes. Shuo et al. dissolves the NH2—(CH2)6—NH2 in aqueous HCl and assumed that the quaternary head group of the +NH3—(CH2)6—NH2 so formed becomes (a) electrostatically attached to the negatively charged MMT−, and (b) will also react with an isocyanate groups to produce a urea linkage: MMT−-NH2+—CONH—. They further postulated that the (non-quaternized) —NH2 end group reacts with another isocyanate group and yields a further urea linkage: —NH—CO—NH—. In other words, Shuo et al. regard their OmMMT as a MMT-tethered chain extender. The authors illustrated their proposition with chemical equations (see Scheme 2 in the Shuo et al. reference). However, this proposition is flawed because the relatively highly acidic —NH3+ will preferentially protonate the basic —SiO— sites in the MMT (—NH3++—SiO—=—NH2+—SiOH+—) and the ionic attachment between the modifier and MMT will diminish if it will occur at all (i.e., the organic amine will likely not be bound to the MMT). Indeed, Shuo et al. recovered polymer by solvent extraction from their nanocomposite, which indicates a lack of attachment between the MMT and their modifier. Further, Shuo et al. fails to demonstrate direct attachment between the MMT and the modifier, for example, by spectroscopic analysis or other means.
Tien and Wei, Macromolecules, 34, 9045, (2001) employs a similar strategy to enhance the properties of polyurethanes. In that reference, Tien and Wei quaternized mono-, di-, and tri-hydroxyl amino alcohols (3-amino-1-propanol, 3-amino-1,2-propane diol, and tris(hydroxymethyl) amino methane) with HCl, and used these quaternized amines as swelling (exfoliating) agents with MMT. The —NH3+ groups were assumed to be ionically connected to the MMT and the free —OH groups to react with isocyanates. However, in these systems, just as with Shou et al.'s systems (see above), the —NH3+ will preferentially protonate the basic —SiO— sites in MMT, which will severely diminish if not altogether eliminate the ionic linkage between the modifier and MMT.
Thus, there is a need in the art for the production of polyurethane nanocomposites that uses organically-modified MMT or other layered clays to improve the mechanical properties of the polyurethanes.